1. Field of the Invention
The present invention generally relates to a wireless communication system and a wireless communication apparatus included therein that communicates data, and more particularly, to a wireless communication system and a wireless communication apparatus included therein having one or more antenna.
2. Description of the Related Art
A conventional wireless communication apparatus generally has an antenna for transmission and an antenna for reception, or a single antenna for transmission and reception. A wireless communication apparatus having multiple antennas using space-diversity reception techniques is known. A wireless communication system is also known as a Multiple Input Multiple Output (MIMO) system in which both a transmitting apparatus and a receiving apparatus are provided with multiple antennas, and the transmitting apparatus and the receiving apparatus communicate using space division multiplexing techniques in accordance with the number of antennas. Multiple data streams are transmitted between the multiple antennas of the transmitting apparatus and those of the receiving apparatus using space division multiplexing techniques through independent multiple channels. Various techniques of MIMO are under development (see reference documents No. 1 through 4, for example).
Technique for a MIMO wireless communication system is proposed in which each data stream is adaptively controlled such that the transmission rate thereof becomes optimal (see reference document No. 5, for example). A wireless communication system using Wideband-Code Division Multiple Access (W-CDMA) is known in which data is transmitted by modulating and multiplexing each channel with spreading code. This W-CDMA technique has been already used for wireless communications between a cellular phone and a base station, for example.
High Speed Downlink Packet Access (HSDPA) is defined that enables the W-CDMA system to transmit data at 14 Mbps or less through a downlink. This system employs adaptive encoding modulation system for packet transmission, in which Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (16 level QAM), for example, are adaptively switched, such that transmission rate can be adapted to the condition of wireless transmission channel.
The HSDPA employs Hybrid Automatic Repeat Request (H-ARQ). For example, if a mobile terminal detects an error in data received from a base station, the mobile terminal requests the base station to re-transmit the data. The base station re-transmits the data in response to receipt of the request. The mobile terminal uses both of the original data and re-transmitted data to perform error correction.
An example of wireless channel used in the HSDPA system includes High Speed-Shared Control Channel (HS-SCCH), High Speed-Physical Downlink Shared Channel (HS-PDSCH), (High Speed-Dedicated Physical Control Channel).
The wireless channels HS-SCCH and HS-PDSCH are common channels (down)ink) from a base station to a mobile terminal of a mobile wireless communication system. HS-SCCH is a control channel through which various parameters related to data transmitted through HS-PDSCH are transmitted. An example of the parameters includes modulation type information indicating modulation method by which data is transmitted using HS-PDSCH, the number of spreading codes, the pattern information of rate matching processing performed on transmission data.
HS-DPCCH is a individual control channel (uplink) from the mobile terminal to the base station of the mobile wireless communication system. The HS-DPCCH transmits ACK signal or NACK signal from the mobile terminal to the base station, which indicates whether data is correctly received through the HS-PDSCH. For example, if CRC error is detected in the received data, a NACK signal is transmitted to the base station. In response to receipt of the NACK signal, the base station re-transmits the data. HS-DPCCH is used to periodically transmit the result of measurement on the condition of received signal from the base station (Signal to Interference Patio (SIR), for example) as a Channel Quality Indicator (CQI). The base station determines whether the condition of downlink wireless channel is satisfactory. If satisfactory, the base station changes the modulation method to a modulation method with which data can be transmitted at higher speed. If not satisfactory, the base station changes the modulation method to a modulation method with which data is transmitted at lower speed.
FIG. 15 is a schematic diagram for explaining the channel structure of the HSDPA. In FIG. 15, CPICH, P-CCPCH, HS-SCCH, HS-PDSCH, and HS-DPCCH are schematically shown. Common Pilot Channel (CPICH) and Primary Common Control Physical Channel (P-CCPCH) are downlink common channels. The CPICH is used for channel estimation, cell search, and timing of other downlink physical channels in the same cell. The CPICH is a channel through which so-called pilot signals are transmitted. The P-CCPCH is a channel through which notice information is transmitted. The HS-SCCH, HS-PDSCH, HS-DPCCH are control channels described above. The above CQI and ACK/NACK are transmitted through the HS-DPCCH.
Fifteen slots form one frame (10 ms). Because the CPICH is used as the reference of timing, the heads of the P-CCFCH and HS-SCCH match the head of the CPICH in timing. However, the head of the HS-PDSCH lags behind the other channels by two slots. This lag allows a mobile terminal to receive information required for the demodulation of the HS-PDSCH. That is information related to modulation and spreading code is transmitted through the HS-SCCH before the demodulation and decoding of the HS-PDSCH. In HS-SCCH and HS-PDSCH, three slots form one sub-frame.
According to 3GPP TS25.212 v.5.7.0, the following information are transmitted through the HS-SCCH:
(a) Channelization Code Set Information (Xccs): 7 bits, information of spreading code used for the HS-DSCH;
(b) Modulation Scheme Information (Xms): 1 bit, demodulation technique used for the HS-DSCH;
(c) Transport-Block Size Information (Xtbs): 6 bits, the block size of transmission data converted into error correction code;
(d) Hybrid-ARQ Process Information (Xhap): 3 bits, process number for re-transmission;
(e) Redundancy and Constellation Version (Xrv): 3 bits, a parameter for rate matching;
(f) New Data Indicator (Xnd): 1 bit, information indicating new data; and
(g) UE Identity (Xue): 16 bits, user identification information.
The information transmitted through the HS-SCCH has 37 bits. This information allows the mobile terminal to learn the parameter of modulation technique, spreading code, error correction used in the HS-DSCH. As a result, the mobile terminal can demodulate and decode HS-DSCH based on these parameters.
The (a) Xccs indicates spreading code used for the transmission of data through HS-PDSCH, such as a combination of the number of multicodes and code offset, The (b) Xms indicates which modulation technique, QPSK or 16 level QAM, is used by “1” and “0”. The (c) Xtbs is data for calculating the size of data transmitted by one sub-frame of HS-PDSCH. The (d) Xhap indicates the process number of H-ARQ, which is a serial number of transmitted data block. When data is re-transmitted, the same process number as the previous transmission data is used.
The (e) Xrv indicates the redundancy version parameter and constellation parameter of HS-PDSCH in re-transmission. A determination is made of whether the parameters are updated depending on the case of transmission and re-transmission. The (f) Xnd indicates whether a transmission block of HS-PDSCH is new block or re-transmitted block. If new block, Xnd alternates between “1” and “0”. If re-transmitted block, Xnd is the same as that of the original block. The (q) Xue is the identification information of a mobile terminal (user).
The reception of HS-SCCH allows the mobile terminal to know the parameters of modulation technique, spreading code, and error correction used in HS-PDSCH and to demodulate and decode the HS-PDSCH.
The following documents are cited for reference: (1) Japanese Patent Laid-Open Application No. 2004-135304; (2) Japanese Patent Laid-Open Application No. 2003-38779; (3) Japanese Patent Laid-Open Application No. 2003-332963; (4) Ari Hottinen, Olav Tirkkonen, Risto Wichman, “Multi-antenna Transceiver Techniques for 3G and Beyond”; (5) 3GPP TS 25.211 v5.5.0 (3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical channels and mapping of transport channels onto physical channels (FDD)) (6) 3GPP TS 25.213 v5.5.0 (3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Spreading and modulation (FDD)); and (7) 3GPP TS 25.214 v5.7.0 (3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD)).
The number of bits required for the above parameters (a) Xccs, (b) Xms, (c) Xtbs, (d) Xha, (e) Xrv, (f) Xnd, and (g) Xue, in the case in which the HSDPA system is adapted to the MIMO system, are shown in FIG. 16 for a mode in which MIMO is not used, a N multiplexing 1 packet mode in which MIMO is used, and a N multiplexing stream independent mode in which MIMO is used. In FIG. 16, N means the number of MIMO multiplexing, and [log 2(N)] means the least integer equal to or more than N.
The above N multiplexing 1 packet mode is a mode in which a packet is divided into data, and each item of the data is transmitted through one of N paths formed by the MIMO multiplexing. The above N multiplexing stream independent mode is a mode in which different packets are transmitted through N paths, respectively, formed by the MIMO multiplexing.
In the MIMO N multiplexing stream independent mode, different transmission technique can be applied to each path. As a result, if the number of MIMO multiplexing N increases, the amount of information considerably increases. If the resource (time, frequency, and spreading code, for example) is allocated in advance in accordance with the estimated maximum amount of control information, when the MIMO multiplexing N increases, remaining resource is reduced, which may result in decrease in throughput. On the other hand, if MIMO multiplexing is used for increasing the number of bits transmitted as the control information, even a user in a wireless transmission environment where MIMO multiplexing is not suitable is required to receive control information containing many bits, which may increase error rate of received control information.
If the power of control channel is increased in order to improve transmission quality, interference with other channels may consequently increase, which results in reduction in the throughput of a wireless communication system. In addition, not all mobile terminals in a mobile phone system usually support MIMO function. There are mobile terminals supporting MIMO function and mobile terminals not supporting MIMO function in such a mobile phone system, which means that the control channel can not always be transmitted using MIMO system. Furthermore, the traffic channel and the control channel need to be controlled to optimize transmission quality. However, if it is desired to increase the information amount of control channel by changing the rate of error correction encoding without using MIMO, the control of the traffic channel and the control channel becomes very difficult because the transmission rate of MIMO multiplexed traffic channel is increased.